1. Filed of the Invention
The present invention relates to an encoder, often utilized in the fields of industrial machines and measurement devices, for detecting the direct moving state or the rotating state of an object to be measured, and an apparatus having the encoder.
2. Related Background Art
Conventionally, in, e.g., a magnetic disk or an optomagnetic disk, a recording/reproduction magnetic head as a movable portion must be precisely moved along tracks. In particular, as the interval between adjacent tracks becomes smaller, the magnetic head must be aligned and fixed above an objective track with higher precision. Conventionally, aligning control at that time is performed by an aligning apparatus using an encoder.
FIG. 15 is a schematic view showing a principal part of a magnetic disk recording system when a conventional aligning apparatus is used.
In FIG. 15, a magnetic head 92 performs a recording/reproduction operation on a magnetic disk 91, and is arranged on one end of an arm 94. An actuator 93 moves the arm 94 coupled to the magnetic head 92, thereby aligning the magnetic head 92 on a target track on the magnetic disk 91. A linear encoder 95 detects position information such as the moving amount or position of the arm 94. More specifically, the position information of the magnetic head 92 is obtained by the linear encoder 95.
FIG. 16 is a graph for explaining output signals of two phases obtained from the linear encoder 95. These output signals are obtained by an encoder shown in FIG. 21, as will be described later.
As shown in FIG. 16, an A phase signal 61, and a B phase signal 62 are output in correspondence with the displacement of the actuator. The phase difference between the two signals is set to be 90.degree.. One of the signals 61 and 62 is selected, and is processed in a feedback loop like a processing circuit shown in FIG. 18.
In FIG. 18, one of the A and B phase signals 61 and 62 from the linear encoder 95 is selected by a switch 9, and the selected signal is input to a driving circuit 7 through a filter 8. The actuator 93 is driven on the basis of a signal from the driving circuit 7, thereby determining the position of the arm 94, i.e., aligning the magnetic head 92. The aligning operation is performed based on the zero-crossing point of the output signal from the encoder 95 at that time. The zero-crossing point means a point where the output signal from the encoder 95 has a zero voltage.
In this manner, conventionally, the displacement of the arm 94 as a movable portion, and the output signals from the encoder 95 are set to have the relationship shown in FIG. 16, and the arm, i.e., the magnetic head is aligned by utilizing the output signals from the encoder at that time.
In general, points where an aligning operation can be attained based on signals obtained from the encoder 95 are only the zero-crossing points of the A and B phase signals 61 and 62. By moving the zero-crossing points of the two phase signals, the aligning operation is performed at an arbitrary position.
FIG. 19 is a schematic diagram of a signal processing circuit for the two phase signals at that time. The circuit shown in FIG. 19 has an adder 10 for adding an offset signal 11 to an output voltage from the encoder 95.
A conventional method of aligning a movable portion to an arbitrary position using the offset signal 11 and the two phase signals 61 and 62 will be described below with reference to FIG. 19 showing the signal processing circuit, and FIG. 16 showing output signal waveforms.
In FIG. 16, one zero-crossing point 14 of the B phase signal 62 will be referred to as a P point hereinafter. The movable portion is aligned with the P point, and is moved in a positive direction (a right direction in FIG. 16). Thus, an aligning feedback loop shown in FIG. 19 is assumed to normally operate according to the polarity of the B phase signal 62 shown in FIG. 16, i.e., the inclination of the B phase signal 62 in FIG. 16 as a position signal at the P point 14. Therefore, when an aligning operation is performed at an R point 16, the A phase signal 61 must be inverted by a non-inverting and inverting circuit 12.
Thus, a point to perform an aligning operation is assumed to be the P point 14. At this time, the offset signal 11 is zero. As the magnitude of the offset signal (voltage) 11 is increased in the negative direction, the aligning point is moved in the positive direction. When the aligning point is moved to a Q point 15 (the voltage of the offset signal 11 is -V.sub.c (13)), the A phase signal 61 is selected by the switch 9, and is inverted by the non-inverting and inverting circuit 12.
When +V.sub.c (13) is output as the offset signal, the movable portion can be aligned with the Q point 15 according to the A phase signal 61 in turn. Upon repetition of the above-mentioned operations, the movable portion can be continuously aligned with an arbitrary position.
Encoders can be roughly classified into optical and magnetic encoders. The encoders can also be roughly classified into linear and rotary encoders. Any encoder can obtain two phase outputs having triangular waveforms, as shown in FIG. 16. However, waveforms obtained from the encoder rarely have perfect triangular waveforms shown in FIG. 16, and in practice, often have pseudo sine waveforms approximate to the sine waveforms, as shown in FIG. 17. An encoder, which can provide two phase outputs having perfect sine waveforms, is also available. An aligning operation 10 with an arbitrary position can also be performed based on two phase signals having sine waveforms in the same method as described above. However, in contrast to the triangular waveform, the waveform of, e.g., a B phase signal 62 has different inclinations at a P point 14 and a Q point 15. It is undesirable that the waveform has different inclinations since the loop gain of the feedback loop shown in FIG. 19 then changes.
Furthermore, a voltage at the Q point 15 is closer to the peak voltage of the waveform than that of the triangular waveform. FIG. 20 shows the B phase signal 62 when the aligning operation is performed at the Q point 15. If the movable portion is displaced due to, e.g., a disturbance, and exceeds an S point 17, the feedback loop shown in FIG. 19 performs a positive feedback operation with respect to the Q point 15, i.e., operates separate from the Q point 15, and cannot recover to the Q point 15.
The inclination of the waveform at the Q point 15 in FIG. 17 is smaller by about 30% than that at the P point 14 since the waveform is a sine wave. Thus, a recovery force against a disturbance is small, and hence, the Q point 15 is weaker against disturbance than the P point 14. For this reason, it is difficult and not desirable to perform an aligning operation near the Q point 15 in FIG. 17.
In the aligning apparatus utilizing an encoder, when two phase signals can only be obtained from the encoder, it is difficult to reliably align a movable portion with an arbitrary position using the two phase signals since these signals are weak against, e.g., disturbance.
Some conventional encoders will be described below.
FIG. 21 is a schematic view of the principal part of a conventional linear encoder for detecting the position or amount of movement an object to be measured in a linear direction. In FIG. 21, a light-emitting means 1 has a light source, a collimator lens, and the like, and radiates a collimated beam. A main scale 2 is arranged in association with an object to be measured, and has a plurality of slit-like (rectangular) openings (to be also referred to as windows hereinafter) 2a at a pitch P in a moving direction x. A light-receiving means (detection head) 5 has a mask 3 having two slit-like openings (to be also referred to as windows hereinafter) 3a and 3b, and two sensors, i.e., an A phase photodiode 41 and a B phase photodiode 42 corresponding to the openings 3a and 3b, so as to detect a moving state of the main scale 2.
A light beam emitted from the light-emitting means 1 passes through the openings 2a of the main scale 2, and becomes incident on the A and B phase photodiodes 41 and 42. The main scale 2 has the slit-like windows at the pitch P, and the mask 3 has the slit-like windows 3a and 3b for the A and B phase photodiodes 41 and 42. For this reason, output currents from the A and B phase photodiodes 41 and 42 are proportional to the amounts of light radiated within the windows 3a and 3b of the mask 3.
Assume that a light beam from the light-emitting means is a perfectly collimated beam for the sake of simplicity. When the collimated beam is incident on the main scale 2, it can pass through only the windows 2a of the main scale 2. For this reason, the light-receiving means 5 receives light beams from regions corresponding to the windows 2a of the main scale 2.
FIG. 22 is a schematic plan view of the main scale 2 and the light-receiving means 5. The width (D.sub.1) of each window 2a in the x-direction of the main scale 2, and the width (D.sub.2) of each of the windows 3a and 3b of the mask 3 are half the pitch P of the windows 2a of the main scale 2 (D.sub.1 =D.sub.2 =P/2).
At this time, the output currents I from the A and B phase photodiodes 41 and 42 are proportional to the light amount of light beams radiated within the windows 3a and 3b of the mask 3 (the shape of the window will be referred to as a "light-receiving surface window shape" hereinafter). For this reason, when the position of the main scale 2 relative to the light-receiving means 5 is displaced in the x-direction, the relationship between the displacement and the output currents I from the A and B phase photodiodes (41 and 42) is as shown in FIG. 23.
As shown in FIG. 23, the output currents I from the A and B phase photodiodes (41 and 42) have triangular waveforms 61 and 62. As shown in FIG. 22, the interval in the x-direction between the A and B phase photodiodes (41 and 42) is (n+0.25).times.P(n=1, 2, 3, . . . , and P is the pitch of the openings 2a of the main scale 2). In this manner, the output currents I from the A and B phase photodiodes (41 and 42) have a phase relationship of 90.degree., as shown in FIG. 23. Normally, measurement of a displacement or speed and aligning control of a movable portion (not shown) arranged in association with the main scale 2 are performed using the A and B phase outputs (61 and 62).
For example, a counter is driven by a rectangular waveform obtained by comparing the A and B phase outputs (61 and 62) at the center of waveforms, thereby measuring a displacement. When the A and B phase outputs (61 and 62) are differentiated over time, the speed can be detected. At this time, the waveforms of the A and B phase outputs (61 and 62) largely influence a speed signal. For example, when the waveform at that time changes from a predetermined pattern, its differentiated waveform also changes, and the speed signal changes accordingly.
When the waveform changes in a case wherein one of the A and B phase outputs (61 and 62) is used as a position feedback signal to perform aligning and measurement, a loop gain of a feedback system at that time undesirably changes. In this manner, when the waveforms of the A and B phase outputs (61 and 62) change from predetermined patterns, detection precision of a moving state deteriorates.
However, in general, in FIG. 21, a light beam from the light-emitting means 1 is not a perfectly collimated beam. For example, a perfectly collimated waveform cannot be obtained due to the manufacturing precision of a lens, a light source which is not a spot light source, and the like. In addition, when a light beam passes through the openings 2a of the main scale 2, it diverges due to diffraction.
If a light beam emitted from the light-emitting means 1 is a perfectly collimated beam, a brightness pattern formed on the surface of the light-receiving means 5 becomes as shown in FIG. 25. However, since the light beam is not a perfectly collimated beam, the brightness pattern often becomes as shown in FIG. 26. In the case of FIG. 25, the A and B phase outputs (61 and 62) have triangular waveforms, as shown in FIG. 23. In contrast to this, in the case of the brightness pattern shown in FIG. 26, the A and B phase outputs (61 and 62) have waveforms, as shown in FIG. 24.
In addition, the brightness pattern formed on the surface of the light-receiving means 5 also changes depending on the interval (distance) between the main scale 2 and the light-receiving means 5. For example, even when a light beam emitted from the light-emitting means 1 is not a collimated beam, if the distance is assumed to be zero, the brightness pattern formed on the surface of the light-receiving means 5 is as shown in FIG. 25.
However, it is difficult in terms of the arrangement to set the distance to be zero or almost zero. The distance between the main scale 2 and the light-receiving means 5 often has a variation caused by assembling precision. For this reason, the brightness pattern on the surface of the light-receiving means 5 changes, and the output waveforms from the photodiodes 41 and 42 vary, resulting in measurement errors.
FIG. 27 is a schematic view of another conventional encoder. The encoder shown in FIG. 27 has substantially the same arrangement as that of the encoder shown in FIG. 21, except that the positions of openings (windows) 31a and 31b of a mask 31 provided to the light-receiving means 5 are different from those in the encoder shown in FIG. 21.
In the encoder shown in FIG. 27, when the light-receiving means 5 is pivoted through an angle .DELTA..sub.y about the y-axis as a mounting error upon assembling, the phase relationship between A and B phase outputs changes almost in proportion to the angle .DELTA..sub.y. Such a displacement of the phase relationship also occurs when the light beam from the light-emitting means 1 is not a collimated beam in the encoder shown in FIG. 21. More specifically, the phase relationship between the A and B phase outputs is displaced.
FIG. 28A is a schematic view of an encoder in which the displacement of the phase relationship between the A and B phase outputs caused by the assembling error is corrected. In the encoder shown in FIG. 28A, the shape and positions of openings (windows) of a mask 32 arranged in front of the light-receiving means 5 are properly determined. More specifically, the A and B phase photodiodes 41 and 42 are arranged in the z-direction like in the encoder shown in FIG. 27. In FIG. 28A, furthermore, windows for inverted (or inversion) A and B phase photodiodes 43 and 44 are arranged in the diagonal direction of the A and B phase photodiodes 41 and 42.
FIG. 28B shows the window state at this time. As shown in FIG. 28B, the windows are arranged so that the outputs from the inverted A and B phase photodiodes (43 and 44) are respectively inverted to those from the A and B phase photodiodes (41 and 42). More specifically, when the width of each window 2a of the main scale 2 is given by D/2(P=D), the interval in the x-direction between the A and B phase photodiodes 41 and 42 is given by (n+0.25).times.P(n=0, 1, 2, 3, . . . ), the interval between the A phase photodiode 41 and the inverted A phase photodiode 43 is given by (n+0.5).times.P(n=0, 1, 2, 3, . . . ), and the interval between the B phase photodiode 42 and the inverted B phase photodiode 44 is given by (n +0.5) .times.P(n=0, 1, 2, 3, . . . ).
With this arrangement, the phase relationship error caused by a pivoting error about the y-axis upon assembling is corrected. The reason for this will be described below.
An operation will be described first. The outputs from the photodiodes 41, 42, 43, and 44 shown in FIGS. 28A and 28B are input to a processing circuit shown in FIG. 29. At this time, an A phase output 61 from the A phase photodiode 41 and an inverted A phase output 63 from the inverted A phase photodiode 43, and a B phase output 62 from the B phase photodiode 42 and an inverted B phase output 64 from the inverted B phase photodiode 44 are respectively differentially amplified by differential amplifiers 7. The difference between the A phase output 61 and the inverted A phase output 63, i.e., an A--inverted A phase output 65, and the difference between the B phase output 62 and the inverted B phase output 64, i.e., a B--inverted B phase output 66, become two phase sensor outputs.
At this time, in the encoder shown in FIG. 28A, as the first advantage, the influence of a change in angle .DELTA..sub.y of a relative mounting error between the main scale 2 and the light-receiving means 5 or of a change in distance between the main scale 2 and the light-receiving means 5 with respect to the phase relationship between the two phase outputs is much smaller than that in the encoders shown in FIGS. 21 and 27. As the second advantage, since the final outputs are differential outputs, electrical noise is small. As the third advantage, symmetry of the output waveforms is satisfactory. These three advantages will be described below.
For the sake of simplicity, the outputs from the photodiodes 41, 42, 43, and 44 are assumed to have sine waveforms. Thus, the output signals can be explained by a vector diagram of alternating current theory. FIG. 30A shows the vector diagram of signals shown in FIG. 29. In FIG. 30A, the outputs 61, 62, 63, and 64 have an ideal relationship therebetween. More specifically, the A and B phase outputs 61 and 62 have a phase relationship of 90.degree. therebetween, the A phase output 61 and the inverted A phase output 63 have a phase relationship of 180.degree. therebetween, and the B phase output 62 and the inverted B phase output 64 have a phase relationship of 180.degree. therebetween. The A inverted A phase output 65 and the B--inverted B phase output 66 have a phase relationship of 90.degree. therebetween.
Assume that the phase relationships of the phase outputs shown in FIG. 30A are shifted. At this time, when all the four phase outputs are shifted by the same angle in the same direction, the phase relationship between the A--inverted A phase output 65 and the B --inverted B phase output 66 maintains 90.degree., as shown in FIG. 30B.
When the phase angle between the A phase output 61 and the inverted A phase output 63 is shifted by the same angle in opposite directions, the phase angle of the A--inverted A phase output 65 is left unchanged, and the same applies to the B phase output 62, the inverted B phase output 64, and the B--inverted B phase output 65.
Most phase change states of phase outputs have the above-mentioned tendencies. At this time, as described above, the phase relationship between the A--inverted A phase output and the B--inverted B phase output is maintained to be 90.degree.. For example, assume that the light-receiving means 5 is pivoted through the angle .DELTA..sub.y about the y-axis in FIG. 28A. In other words, assume that the light-receiving means is pivoted through the angle .DELTA..sub.y about an arbitrary point. At this time, even when the light-receiving means 5 is pivoted about any point, this pivotal movement can be replaced with the sum of rotations about almost the centers of the four photodiodes 41, 42, 43, and 44, and their parallel movements.
In the phase change states of the phases in parallel movements, since all the phases are shifted by the same angle, the phase relationship between the two phase signals, i.e., the A--inverted A phase output and the B--inverted B phase output as the final outputs is maintained to be almost 90.degree..
On the other hand, upon rotations about almost the centers of the four photodiodes 41 to 44, the A phase output and the inverted A phase output, and the B phase output and the inverted B phase outputs are shifted by the same angle in opposite directions, respectively. For this reason, the phase relationship between the two phase signals, i.e., the A--inverted A phase output and the B--inverted B phase output as the final outputs is maintained to be almost 90.degree..
The above-mentioned condition is satisfied when each phase waveform is not only a sine wave but also not a sine wave, i.e., its fundamental wave, and the same effect as described above can be obtained in this case.
FIG. 31A shows a case wherein the A phase output 61 has a waveform, which is asymmetrical with respect to a zero voltage in the y-direction, and hence, the interval between zero-crossing points is not constant. The zero-crossing point means an intersection between an output waveform from the encoder and a center voltage (0 V). The above-mentioned phenomenon occurs due to asymmetrical bright and dark levels of an upper brightness pattern of the light-receiving means 5 when the width (D.sub.1) of each window of the main scale 2 is not precisely equal to 1/2 the pitch P. The aligning operation is performed based on zero-crossing points in aligning control of a movable portion. For this reason, if the zero-crossing points are not arranged at equal intervals, an aligning error occurs.
As shown in FIG. 31B, the inverted A phase output 63 is obtained by shifting the phase of the A phase output 61 by 180.degree., and is not obtained by merely inverting it. As a result, the A--inverted A phase output 65 has zero-crossing points arranged at equal intervals, and has a symmetrical waveform with respect to the y-direction, as shown in FIG. 31C.
FIG. 32 shows a case wherein the phase difference between the A phase output 61 and the inverted A phase output 63 is slightly shifted from 180.degree.. In this case, the A--inverted A phase output 65 also has zero-crossing points arranged at equal intervals therebetween, and a symmetrical waveform with respect to the y-direction.
As described above, the encoder shown in FIG. 28 has many advantages. However, this encoder suffers from the following problem. That is, the output waveforms from the photodiodes change according to the distance between the main scale 2 and the light-receiving means 5.
If the A phase output and the inverted A phase output stably have predetermined waveforms and if the phase difference therebetween is shifted from 180.degree., the A--inverted A phase output as the difference between the two outputs have a different waveform from the two waveforms. For example, when triangular wave signals can be obtained from the A phase output and the inverted A phase output, if the phase difference therebetween is shifted from 180.degree., the waveform of the A--inverted A phase output does not become a triangular waveform accordingly. Thus, detection precision of the moving state of the main scale 2 is undesirably deteriorates.